1. Field of the Invention
The present invention relates to a technique for decoding a multiple Moving Picture Experts Group-2 (MPEG-2) encoded Transport Stream (TS), and more particularly to an MPEG-2 decoding system and method for extracting Program Specific Information (PSI) from a transport stream in order to effectuate a channel change.
2. Description of the Related Art
An MPEG-2 system handles two kinds of multiplexed bit streams: a program stream and a transport stream. The program stream is used in a multiplexing scheme where a single broadcast program, including video, audio and captioning data, is transmitted in an error-free channel environment, or when an error correction function of a medium such as a CD is utilized without any alteration thereof. The transport stream is used in a multiplexing scheme where a number of broadcast programs are simultaneously transmitted in a channel environment where there is an error. For example, the program stream is used when storing a single program of a video CD or the like, whereas the transport stream is used when performing digital broadcasting of a plurality of programs using a satellite.
The MPEG-2 system incorporates a packet multiplexing scheme according to a Time Division Multiplexing system. In the packet multiplexing scheme, each of video and audio bit streams is first divided into so-called Packetized Elementary Stream (PES) packets of suitable lengths. The PES packets have an upper size limit of 64 KB to cope with various applications, and each packet size may be fixed or variable. A variable transfer rate and discontinuous transmission are also allowed for the PES packets. The PES packets are multiplexed into a single bit stream to produce the program or transport stream described above.
The packet size highly depends on a transmission channel or medium. For example, 53-byte packets (cells) are used in an ATM (Asynchronous Transfer Mode) protocol in Broadband Integrated Services Digital Networks (BISDN). A header in such a packet, containing basic information thereof, is 5 bytes, and thus an actual payload is 48 bytes. The transport stream packet (hereinafter abbreviated to a “transport packet”) has a fixed size of 188 bytes in an ATM system. The first 4 bytes of the transport packet includes a header information, and the remaining 184 bytes includes a payload carrying video or audio data. The transport packet is transmitted while being carried in 4 ATM cells. If one byte of the 48 payload bytes of each ATM cell is used for an ATM Adaptation Layer (AAL), the actual payload is 47 bytes.
In operation, each broadcast station multiplexes and outputs transport packets. Multiplexed transport packets from a number of broadcast stations are then multiplexed into a single bit stream for a subsequent transmission via a single repeater. Digital satellite broadcasting employs a multiplexing scheme using Time Division Multiplexing (TDM) and Frequency Division Multiplexing (FDM) protocols. Each repeater has a bandwidth of 27 MHz and is operated according to the FDM scheme, and multiple broadcast stations may share a single repeater. A transport stream carried by each repeater is subjected to Reed Solomon coding and convolution coding for error correction, and transmission thereof between a satellite and the ground is performed through Quadrature Phase Shift Keying (QPSK) modulation.
At the receiving end, the reverse procedure of that described above is performed when a received transport stream is decoded. First, a repeater carrying a broadcast signal to be received is selected, and then QPSK-demodulation and error correction are performed on the broadcast signal. The resulting signal is a bit stream into which a number of broadcast signals are multiplexed. Thus, only transport packets of a broadcast station to be received are first selected from the multiplexed bit stream, and video and audio packets thereof are decoded by the video and audio decoders in order to reproduce image and audio signals, respectively.
As described in the International Standard Organization/International Electro-technical Commission (ISO/IEC) 13818, the decoding system includes an elementary stream decoder, including at least one MPEG-2 audio decoder and at least one MPEG-2 video decoder, and an MPEG-2 transport decoder. The decoding system receives MPEG-2 transport streams (bit streams) from a number of service providers (that may have various source types). All the MPEG-2 transport streams may contain a number of programs that can be displayed to users. Thus, switching (i.e., channel change) from one program (or channel) to another program requires switching from one transport stream source to another transport stream source.
If a desired channel exists in a transport stream received by the decoding system, a latency between selection of a new channel and display of the desired channel is influenced based on the time it takes to resynchronize the data of the new channel by the elementary stream decoder and the transport decoder. However, if data of the desired channel does not exist in the currently used transport stream, the desired data must be selected from another transport stream, thus further delaying the switching process. In this case, the transport stream inputted to a transport decoder must be switched to a new transport stream. To parse and decode a program from the new transport stream, the transport decoder must first check Program Specific Information (PSI). Such an operation worsens the latency associated with the channel change, thereby making it difficult to smoothly provide broadcasting services such as digital broadcasting services to users.